Ban empty features in stability annotations

src/librustc_attr/builtin.rs

@@ -21,6 +21,7 @@ enum AttrError {
     MultipleItem(String),
     UnknownMetaItem(String, &'static [&'static str]),
     MissingSince,
+    EmptyFeature,
     MissingFeature,
     MultipleStabilityLevels,
     UnsupportedLiteral(&'static str, /* is_bytestr */ bool),
@@ -41,6 +42,9 @@ fn handle_errors(sess: &ParseSess, span: Span, error: AttrError) {
         AttrError::MissingSince => {
             struct_span_err!(diag, span, E0542, "missing 'since'").emit();
         }
+        AttrError::EmptyFeature => {
+            struct_span_err!(diag, span, E0546, "empty 'feature' attribute").emit();
+        }
         AttrError::MissingFeature => {
             struct_span_err!(diag, span, E0546, "missing 'feature'").emit();
         }
@@ -427,6 +431,10 @@ where
 
                     match (feature, reason, issue) {
                         (Some(feature), reason, Some(_)) => {
+                            if feature.is_empty() {
+                                handle_errors(sess, attr.span, AttrError::EmptyFeature);
+                                continue;
+                            }
                             let level = Unstable { reason, issue: issue_num, is_soft };
                             if sym::unstable == meta_name {
                                 stab = Some(Stability { level, feature, rustc_depr: None });

src/librustc_span/symbol.rs

@@ -1435,6 +1435,11 @@ impl Symbol {
     pub fn to_ident_string(self) -> String {
         Ident::with_dummy_span(self).to_string()
     }
+
+    pub fn is_empty(self) -> bool {
+        // Invalid is "" (the empty string)
+        self == kw::Invalid
+    }
 }
 
 impl fmt::Debug for Symbol {

src/librustdoc/clean/mod.rs

@@ -2348,7 +2348,7 @@ impl Clean<Stability> for attr::Stability {
     fn clean(&self, _: &DocContext<'_>) -> Stability {
         Stability {
             level: stability::StabilityLevel::from_attr_level(&self.level),
-            feature: Some(self.feature.to_string()).filter(|f| !f.is_empty()),
+            feature: self.feature.to_string(),
             since: match self.level {
                 attr::Stable { ref since } => since.to_string(),
                 _ => String::new(),

src/librustdoc/clean/types.rs

@@ -1526,7 +1526,7 @@ pub struct ProcMacro {
 #[derive(Clone, Debug)]
 pub struct Stability {
     pub level: stability::StabilityLevel,
-    pub feature: Option<String>,
+    pub feature: String,
     pub since: String,
     pub deprecation: Option<Deprecation>,
     pub unstable_reason: Option<String>,

src/librustdoc/html/render.rs

@@ -2235,7 +2235,7 @@ fn stability_tags(item: &clean::Item) -> String {
     if item
         .stability
         .as_ref()
-        .map(|s| s.level == stability::Unstable && s.feature.as_deref() != Some("rustc_private"))
+        .map(|s| s.level == stability::Unstable && s.feature != "rustc_private")
         == Some(true)
     {
         tags += &tag_html("unstable", "Experimental");
@@ -2291,25 +2291,25 @@ fn short_stability(item: &clean::Item, cx: &Context) -> Vec<String> {
 
     // Render unstable items. But don't render "rustc_private" crates (internal compiler crates).
     // Those crates are permanently unstable so it makes no sense to render "unstable" everywhere.
-    if let Some(stab) = item.stability.as_ref().filter(|stab| {
-        stab.level == stability::Unstable && stab.feature.as_deref() != Some("rustc_private")
-    }) {
+    if let Some(stab) = item
+        .stability
+        .as_ref()
+        .filter(|stab| stab.level == stability::Unstable && stab.feature != "rustc_private")
+    {
         let mut message =
             "<span class='emoji'>🔬</span> This is a nightly-only experimental API.".to_owned();
 
-        if let Some(feature) = stab.feature.as_deref() {
-            let mut feature = format!("<code>{}</code>", Escape(&feature));
-            if let (Some(url), Some(issue)) = (&cx.shared.issue_tracker_base_url, stab.issue) {
-                feature.push_str(&format!(
-                    "&nbsp;<a href=\"{url}{issue}\">#{issue}</a>",
-                    url = url,
-                    issue = issue
-                ));
-            }
-
-            message.push_str(&format!(" ({})", feature));
+        let mut feature = format!("<code>{}</code>", Escape(&stab.feature));
+        if let (Some(url), Some(issue)) = (&cx.shared.issue_tracker_base_url, stab.issue) {
+            feature.push_str(&format!(
+                "&nbsp;<a href=\"{url}{issue}\">#{issue}</a>",
+                url = url,
+                issue = issue
+            ));
         }
 
+        message.push_str(&format!(" ({})", feature));
+
         if let Some(unstable_reason) = &stab.unstable_reason {
             let mut ids = cx.id_map.borrow_mut();
             message = format!(